The present invention relates, generally, to a communication network or system for transmitting both data and television signals between a plurality of digital computer workstations. More particularly, the invention relates to a communication system that employs an industry standard local area network (LAN) for transmitting data and which makes provision for the selective transmission of television signals between user workstations connected on the LAN as well as between a given workstation and a selected remote workstation which is not connected to the LAN.
As so-called "multimedia" are incorporated into digital computer workstations (also known as desktop computers, personal computers or "PCs") it becomes possible to use such workstations for "videoconferencing" with the users of one or more similar workstations at either nearby or remote locations. To achieve this capability, a workstation must include, as a minimum:
(1) a digital display (e.g., CRT display); PA1 (2) a sound transducer (e.g., loudspeaker); PA1 (3) a digital computer for processing data, connected to supply image and control signals to the digital display for imaging data; PA1 (4) a circuit for converting the video portion of the TV signal into image and control signals compatible with the digital display for displaying a picture on this imaging device; and PA1 (5) an analog amplifier or the like which provides the proper gain and impedance for driving the loudspeaker with the audio portion of the TV signal.
A conversion circuit of the aforementioned type, which digitizes an analog video signal and formats this digitized signal in such a way as to permit display of the video image on a digital computer display, is well known. Such a circuit is available for both the PS/2 computer of International Business Machines Corp. and the MacIntosch computers of Apple Computer Corp. The circuit useable with the PS/2 computers is sold by IBM under the part number PN-34F3087.
If two-way communication with the digital computer workstation is desired, it is necessary also to provide the workstation with a video camera, aimed appropriately at the workstation user, for producing a video signal representing the image of the workstation user, and a microphone arranged to pick up the voice of the workstation user and to generate an analog audio signal in response thereto.
The term "television signal" or "TV signal", as used herein, is intended to mean a conventional NTSC (or other standard) signal which includes both a video and an audio portion. The terms "video signal" and "audio signal" will be used to separately denote only the video portion and audio portion, respectively, of the television signal. As is well known, the video portion, at baseband, lies within a frequency range of 0 to 4.75 MHz whereas the audio portion lies within a frequency of 0 to 15 KHz. As desired, these video and audio signals can be combined and modulated upward to a 6 MHz wide channel within a broadband spectrum of 100 to 550 MHz.
Similarly, a broadband (100-550 MHz) television signal containing one or more active channels can be tuned and demodulated to produce the video and audio portions (signals) of a single television signal at baseband.
As used herein, the term "baseband signals" is intended to define information signals within the frequency range of 0 to 100 MHz. Computer data is normally transmitted by baseband signals. The term "broadband signals" is thus intended to mean information signals at a frequency higher than the highest baseband frequency--typically in the frequency range of 100 to 550 MHz. Television is normally transmitted by broadband signals in the frequency range of 50 to 450 MHz. If the upper limit of the baseband range terminates at 50 MHz, the lower limit of the broadband range can commence at this frequency.
It is known to transmit both baseband and broadband signals on a common "backbone communication network" such as a local area network (LAN) which is connected to a plurality of user workstations. The U.S. Pat. No. 4,885,747 to Foglia discloses a so-called "filter coupler" or "F-coupler" by which baseband signals (data) are transmitted between a backbone network and a given workstation via a twisted pair shielded cable in a balanced mode and broadband (television) signals are transmitted between the same backbone network and the same workstation via the same shielded cable in an unbalanced mode. The disclosure of this U.S. patent is incorporated herein by reference.
With the Foglia system, a number of television programs (satellite television, VCR or a "live" broadcast from a television camera) is "broadcast" to all workstations connected to the LAN (an IBM token ring, in this case) from a coaxial cable through a so-called "tap/combiner". While each workstation can select one from a number of television channels that are broadcast on the broadband frequencies, and while it is even possible for a workstation to broadcast to all other workstations by means of a TV camera or some other program source, it is not possible for any particular workstation to transmit television signals to any particular workstation or workstations on the LAN, or to transmit to any remote workstation not connected to the LAN, thereby to provide true videoconferencing capability.
The U.S. Pat. No. 4,564,940 to Yahata discloses a so-called "broadband network system" which includes a private branch exchange (PBX) for interconnecting a plurality of workstations. However, this system is intended to replace an industry standard local area network (LAN) for the multiplex communication of voice and data. No consideration is given to the special problems encountered by the transmission of television signals.